Among the various semiconductor nanomaterials investigated for photovoltaic applications, one of the more promising candidates for low cost solar cells is the I-III-VI2 family of chalcopyrite nanocrystals, particularly Cu(In,Ga)Se2 (3, 5, 6). Thin film solar cells based on Cu(In,Ga)Se2 has reach photon to electron conversion efficiency as high as 19.9% (13). However, due to the limited supply and ever increasing price of rare metals such as indium and gallium, there's need to find alternative materials with high abundance and low cost. Recently, few selective groups have started investigating Cu2SnZnS4 (CZTS) and Cu2SnZnSe4 (CZTSe) thin films for photovoltaic applications. CZTS and CZTSe are especially attractive because of tin and zinc's natural abundance in the earth's crust and they are of relatively low toxicity. Solar cell based on CZTS has achieved photon to electron conversion efficiency as high as ˜6.7% and expecting further increases (8). Various high-vacuum and non-vacuum based techniques similar to that of Cu(In,Ga)Se2 absorber have been investigated for the deposition of CZTS thin films, such as vacuum co-evaporation and selenization of various precursor layers (1, 2, 9, 15, 16, 18). However, such deposition methods are problematic for high throughput fabrication as evidenced by the slow progress in mass production of Cu(InGa)Se2-based solar cells.
Recently, synthesis and in some cases photovoltaic applications of various semiconductor nanocrystals has been reported, such as Cu2S, (14), (CdTe, (7), Pb(S,Se)(11, 12), and various I-III-VI2 chalcopyrite compounds, such as CuInSe2 (6), Cu(In,Ga)Se2 (3, 10, 17), and Cu(In,Ga)S2 (4, 5). Recent reports demonstrated the ability to synthesize various multinary chalcogenide nanocrystals and inks based on Cu, In, Ga, S, and Se for high efficiency thin film solar cells (4, 5, 6). In such nanoparticles, both In and Ga belong to the same column of the Periodic Table (CAS Version, CRC Handbook Version, CRC Handbook of Chemistry and Physics) and have a valency of three. The inter substitution of In and Ga in the lattice essentially exchanges one element with another having the same valency. By forming nanoparticles of Cu(In,Ga)(S,Se)2, the composition of the films is fixed at length scales equivalent to the nanoparticle size which can lead to device quality absorber films reproducibly while taking the advantages of a solution based process.
There is a need in the art for simple and scalable synthesis of multinary nanoparticles comprising of copper (Cu), zinc (Zn), tin (Sn), sulfur (S), and selenium (Se) suitable for photovoltaic applications. The present invention provides compositions and methods for synthesizing multinary chalcogenide CZTSSe nanoparticles containing Cu, Zn, Sn, S, and Se.